swiftlang/swift

[SR-6979] ThinToThickFunction derived pointers do not need to be retained/released

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#49,527 建立於 2018年2月11日

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bugcompilergood first issueperformance

描述
Previous ID SR-6979
Radar None
Original Reporter @gottesmm
Type Bug
Votes 0
Component/s Compiler
Labels Bug, Performance, StarterBug
Assignee None
Priority Medium

md5: 570977381e8bce3c10e08267eb895f6f

Issue Description:

I was reading some SIL recently:

  debug_value undef : $Error, var, name "$error", argno 1 // id: %0                                                               
  // function_ref thrower()                                                                                                       
  %1 = function_ref @$S8rethrows7throwerSiyKF : $@convention(thin) () -> (Int, @error Error) // user: %2                          
  %2 = thin_to_thick_function %1 : $@convention(thin) () -> (Int, @error Error) to $@callee_guaranteed () -> (Int, @error Error) \
// user: %3                                                                                                                       
  %3 = convert_function %2 : $@callee_guaranteed () -> (Int, @error Error) to $@noescape @callee_guaranteed () -> (Int, @error Er\
ror) // users: %11, %7, %5                                                                                                        
  // function_ref rethrower(_:)                                                                                                   
  %4 = function_ref @$S8rethrows9rethroweryS2iyKXEKF : $@convention(thin) (@guaranteed @noescape @callee_guaranteed () -> (Int, @\
error Error)) -> (Int, @error Error) // user: %5                                                                                  
  try_apply %4(%3) : $@convention(thin) (@guaranteed @noescape @callee_guaranteed () -> (Int, @error Error)) -> (Int, @error Erro\
r), normal bb1, error bb2 // id: %5                                                                                               
                                                                                                                                  
bb1(%6 : $Int):                                   // Preds: bb0                                                                   
  strong_release %3 : $@noescape @callee_guaranteed () -> (Int, @error Error) // id: %7
  ...

%7 should be able to be removed. I imagine though this peephole would need to be done late in the pipeline since it breaks semantic sil invariants.

貢獻者指南

技術棧
swift
領域
performance
議題類型
bug
難度面向新貢獻者的預計實作難度，1 表示很小改動，5 表示專家級工作。
3
預計時間有經驗貢獻者完成調查、實作、測試並準備 pull request 的粗略時間範圍。
half day
活動狀態議題目前的可參與程度：新鮮、活躍、陳舊、阻塞或等待維護者輸入。
fresh
清晰度議題是否清楚說明預期改動、驗收標準和下一步。
clear
前置要求
SIL knowledgecompiler optimization basics
新手友善度1-100 的估計分數，表示該議題對首次貢獻者的友善程度。
70
研究方向
Investigate the SIL optimization passes in the Swift compiler, particularly those that handle thin to thick function. The issue points out that a strong release on a @noescape @callee guaranteed function is unnecessary because thin to thick derived pointers do not need to be retained/released. Look at the SILOptimizer directory (e.g., lib/SILOptimizer/) for existing peephole optimizations (like RedundantLoadElimination or similar) and determine where to add this removal. Consider that the optimization must be placed late in the pipeline to avoid breaking semantic SIL invariants. The issue has no assignee or comments, making it a good candidate for a first contribution.